1. Field of the Invention This invention relates to fluoride ion-leachable glasses and to water-or resin-based dental cement compositions containing such glasses.
2. Related Art
Aqueous poly (carboxylic acid) cement compositions are well known and have been routinely used in dentistry. Such compositions are commonly referred to as glass ionomer cements, and basically comprise (i) a polymer containing free carboxylic acid groups (typically a homo-or co-polymer of acrylic acid) and (ii) an ion-leachable glass such as calcium aluminofluorosilicate glass. In the presence of water, the glass leaches polyvalent metal ions such as aluminum and calcium ions and these serve to cross-link the polymer to give a rigid gelatinous structure. At the same time, silica material in the glass reacts with water to produce silicic acid. As a result of these gel-forming reactions, a cement, suitable for dental use, forms.
A problem with glass ionomer cements in the prior art is that they are radiolucent and thus provide no contrast on x-rays between the cement and surrounding tooth structure. In order to address this issue, it has been proposed to replace calcium in a calcium aluminofluorosilicate glass with strontium and, thereby, produce a glass which may be radiopaque, and still have acceptable properties with regard to strength, hardness, translucency, etc. (U.S. Pat. No. 4,814,362). The use of barium glass or barium sulphate to confer radiopacity on a glass ionomer has also been described (PCT application No. 88,105,651).
Calcium aluminofluorosilicate glass used in the glass ionomer cements contain large quantities of fluoride. Although fluoride lowers the firing temperatures of the glasses, it is also released in a cement composition where calcium aluminofluoro silicate is incorporated. Fluoride, if released in sufficient quantity, imparts carlostatic properties to the cements when used in the repair of carious lesions. However, while calcium aluminofluorosilicate glass is acceptable in terms of fluoride release, it is not desirable in dental use because of its visual opacity and limited radiopacity.
Thus, the requirement of adequate radiopacity and sustained fluoride release makes the development of new glasses desirable. Dental cement compositions containing such glasses should meet such other desirable criteria as: (a) translucency; (b) long shelf life; (c) low solubility in oral fluids; (d) strong adhesion to the tooth; (e) suitable working and setting times; and (f) adequate strength. The present invention provides fluoride ion-leachable glasses and dental cement compositions containing such glasses which meet these criteria.